Detection methods for detecting substances existing near metal micro-particles by fixing metal micro-particles to the surface of a substrate and utilizing localized surface plasmon resonance induced there are known. More specifically, such methods utilize the fact that a characteristic resonance spectrum appears due to localized surface plasmon resonance when light is applied to metal micro-particles of gold or silver and the resonance wavelength depends on the permittivity of the medium existing near the metal micro-particles. For example, when the permittivity of the medium is raised, the absorbance of the resonance peak increases and the position of the resonance peak is shifted toward the long wavelength region. Sensors utilizing this phenomenon are also known. For example, Japanese Patent No. 3452837 discloses a sensor formed by arranging a plurality of metal micro-particles on a substrate so as to make them separated from each other. It is a surface plasmon resonance sensor for detecting the refractive index of the medium existing near the metal micro-particles by measuring the absorbance of light being transmitted through the sensor unit. More specifically, the sensor unit is formed by arranging gold colloids having a diameter of 20 nanometers. The sensor is advantageous relative to ordinary surface plasmon resonance sensors that comprise a unit, which includes a prism and a metal film formed on one of the surfaces of the prism to be brought into contact with a specimen and is adapted to be irradiated with light with various incident angles so as to observe the total reflection angle, in terms of that this invention requires neither a prism nor a special optical system and hence it can be a very compact sensor.
Apart from the above identified sensor, U.S. Patent Application Laid-open No. 2003-0132392 (Japanese Patent Application Laid-open No. 2003-270132) discloses a sensor adapted to induce surface plasmon resonance at a metal thin film having a plurality of apertures arranged periodically, wherein both the apertures and the gaps separating the apertures are smaller than the wavelength of light. According to the above cited patent document, as a result of periodically arranging aperture positions in a well-controlled manner, it can provide a high sensitivity sensor with a narrower resonance spectrum width and a higher resonance peak than micro-particle type sensors not having such a position control feature.
J. Phys. Chem. B 2004, 108, 109-116 describes a study on the sensitivity of the sensors realized by using metal nano-dots arranged on a substrate to induce localized surface plasmon and having a triangular profile and those having a spherical profile. The paper says that triangular metal nano-dots provide a sensitivity level higher than spherical ones.
While the surface plasmon resonance sensors disclosed in the above listed prior art documents are useful, they do not necessarily provide a satisfactory detection sensitivity level in affinity assays such as immuno assays utilizing the specificity of antigen-antibody reactions.